With rapid developments in recent network systems, attention has focused on a multicast communication for simultaneously transmitting, to a plurality of terminals, so-called multimedia data including video data, audio data and the like such as a network game, a teleconference, streaming delivery and the like.
Also wireless communication systems have advanced toward broadband deployment, and 3GPP (3rd Generation Partnership Project), which is a standardization body, has laid down a specification for a multicast system as MBMS (Multimedia Broadcast Multicast Service).
Additionally, 3GPP is laying down next-generation specifications for LTE (Long Term Evolution), and LTE-A (LTE-Advanced) implemented by further developing LTE. LTE-A is compatible with LTE, and may perform transmission and reception operations by simultaneously combining a plurality of frequency bands in order to improve a transmission rate.
A technique disclosed hereafter takes LTE and LTE-A as examples. However, the technique is not limited to LTE and LTE-A.
A multicast delivery system includes a server for managing multicast data, a gateway for delivering the multicast data to a plurality of base stations, the plurality of base stations for transmitting data to mobile terminals, and the mobile terminals receiving data.
Additionally, for LTE, introduction of MBSFN (MBMS Single Frequency Network) that may expand a coverage area with improvements in reception quality of mobile terminals by transmitting the same data at the same timing with the same frequency resources used by a plurality of base stations is in study.
A mobile terminal that receives a multicast service receives multicast data by selecting a service needed for the local terminal based on schedule information notified from a base station.
The mobile terminal may maintain the multicast service by executing a handover process for switching a base station from which the mobile terminal receives the data while moving. However, if a base station at a switching destination delivers the multicast service only at one frequency, the mobile terminal may not be able to perform a handover in some cases depending on the state of the base station. For example, there may be cases where the number of mobile terminals accommodated in a frequency band at the switching destination becomes too large to accommodate a new mobile terminal.
The following prior art document is disclosed as a conventional technique related to the technique disclosed by the present application.    Patent Document 1: Japanese National Publication of International Patent Application No. 2005-508584